A railroad is a series of ties lying transverse to the line of train travel and a pair of steel rails lying atop the ties and along the line of train travel. Traditionally, the railroad ties have been wooden and placed in contact with the earth, but decomposition due to such earth contact has been a primary cause of wear in wooden railroad ties. Also, creosote typically used in such wood railroad ties is of environmental concern. More recently, concrete railroad ties have replaced wooden railroad ties as concrete is less affected by earth contact and, therefore, lasts longer. Additionally, the rail is easier to remove and replace when used in conjunction with concrete ties. Finally, concrete railroad ties provide a stiffer, and therefore more desirable, overall railroad track configuration.
Use of concrete railroad ties presents a different source of railroad tie wear as compared to use of wooden railroad ties. The wood-to-steel interface for a wooden tie railroad does not present as significant source of abrasion to the railroad tie as does the concrete-to-steel interface for a concrete railroad tie. In other words, wood is more resilient than concrete and better withstands the pounding forces imposed by the steel rail than does the less resilient concrete. To mount a rail upon a concrete tie, a spring fastener couples the rail and tie, and bears downward upon the rail to maintain the rail against the rail seat of the tie. A polymer pad lies between the tie and the rail to prevent direct contact between the steel of the rail and the concrete of the tie. Despite placement of such polymer pad or similar material, however, rail seat abrasion remains a problem in concrete ties.
Rail seat abrasion can significantly deteriorate the performance and dramatically shorten the life of the railroad tie. Specifically, deterioration of the rail seat can resulting in loosening of the rail from the railroad tie. The spring fastener must bias the rail against the rail seat under a specified magnitude of spring tension as a rail mounting force. When the rail seat wears, however, this affects the amount of tension in the spring fastener and the required magnitude of rail mounting force may not be achieved. For example, a typical fastening spring develops approximately 2650 pounds per square inch (psi) at the rail seat, but for every millimeter of rail seat lost to abrasion, as much as 100 psi of fastening force is lost. Ultimately this decreased holding ability of the fastener system allows the rail to slide atop and impact the concrete tie resulting in accelerated rail seat abrasion, pull-apart, and thermal misalignments.
An additional concern is that abrasion of the concrete tie surface exposes the aggregate within the tie to environmental conditions, moisture, sand and other contaminants thereby further accelerating rail seat abrasion. Finally, the abraded rail seat also forms a water collecting pocket contributing to further degradation of the concrete tie.
The problem of abrasion in concrete railroad ties has manifested itself most notably in the concrete railroad ties used in the Northwest portion of North America. Concrete railroad ties have been used in Europe for many years, but without acute rail seat abrasion. Concrete railroad tie abrasion may be due to the combined affects of the extremely high, as compared to Europe, wheel loadings commonly handled by North American railroads operating in rugged, steep mountainous areas with numerous restrictive curves. At this time, however, the causes of concrete tie abrasion are not clearly understood.
Rail seat abrasion may be related to excessive compressive stresses developed in the system which attaches the rail to the concrete tie. Another potential cause of rail seat abrasion may be moisture collecting under the polymer pad at the rail seat surface which may then be absorbed into the concrete. Repeated freezing and thawing of this absorbed water results in cracking and deterioration of the concrete structure. Other possible sources of rail seat abrasion may be particular manufacturing defects; temperature variation patterns; railroad arrangements such as curvature, grade and banking; and the nature of the pad interposed between the rail and the rail seat.
Whatever the cause of rail seat abrasion in concrete railroad ties, there exists a need to better understand the phenomenon of rail seat abrasion so that a solution and preventative measures may be taken. The fact the European railroads experience less rail seat abrasion than do North American railroads suggests that rail seat abrasion can at least be explained and predicted for preventative maintenance, and hopefully avoided altogether.
Hence, concrete rail tie abrasion, especially in North American railroads, is an area of concern to the railroad community. Careful monitoring and study will yield solutions to concrete railroad tie abrasion and thereby avoid acute deterioration of concrete railroad ties.
Measurement of concrete tie abrasion has been conducted in an attempt to monitor the wear of concrete ties and better understand the nature of such abrasion. One method of rail seat abrasion measurement is by direct manual measurement. In this method of rail seat abrasion measurement, the rail seat is exposed and a measuring frame is placed adjacent the rail seat abrasion site. More specifically, the measuring frame lies about the outer periphery of the abrasion site upon the upper surface of the tie and a dial meter is used to measure a distance from a reference point on the measuring frame to the abraded surface of the rail seat. While some form of registration of the measuring frame and rail seat has been used, this method of measurement has proven unreliable for the purpose of accurate rail seat abrasion measurement.
One major setback in such abrasion studies, therefore, has been an inability to provide reproducible abrasion measurements. In other words, a reliable method of rail seat abrasion measurement must provide the same wear measurements for a given rail seat on repeated measurements. Such reproducible abrasion measurements are essential to reliable long-term study of rail seat abrasion measurements. More particularly, in order to accurately determine the rate of rail seat abrasion, one must collect measurements of rail seat abrasion and later return, e.g. after a given period of railroad service and exposure to environmental conditions, and collect a second set of measurements for the same abrasion sites. By using a reproducible method of rail seat abrasion measurement, the amount of wear can be inferred directly from measurement differences.
A second concern with such manual rail seat abrasion measurement is the danger associated with an upraised continuous length railroad rail. More particularly, to accomplish rail seat measurement, it is necessary to expose the rail seat. Such exposure requires dismounting of the fastening system and lifting of the rail by use of a truck-jack above the rail seat. In a continuous length rail system, the rail can be raised only approximately six inches. This leaves little room at the exposed rail seat for measuring of the abrasion site. Furthermore, such upraised continuous length rails present a hazard to workers, especially if workers were required to perform manual measurement steps underneath the upraised rails. Indeed, because of the unpredictability of such upraised, continuous length rails, workers should never place their hands between an upraised rail and the rail seat. Such hazards are described in all railroad rule books.
Another problem encountered in measurement of rail seat abrasion is the need to quickly obtain measurements. Because the rail must be lifted some distance above the tie in order to expose the rail seat, the section of railroad under inspection is unavailable for train traffic during measurement activities. For this reason, any method of rail seat abrasion measurement must be not only accurate and reproducible, but also rapid in order to minimize the time during which the railroad is unavailable for service.
Accordingly, it would be desirable to provide a method of concrete tie abrasion measurement which is reproducible per individual railroad tie over long separated data collection operations in order to accurately determine the amount of rail seat abrasion. Such accurate methods of rail seat abrasion measurement will provide a better understanding of the phenomenon of rail seat abrasion in concrete ties, and therefore provide a basis for preventing or correcting the problem of rail seat abrasion in concrete ties, or at least provide a basis for predicting concrete tie longevity.
Furthermore, it is desirable that a method of rail seat abrasion measurement be rapid to avoid excess system downtime, be performed with a minimum amount of clearance between the concrete tie and the rail such that the rail need only be lifted a minimum distance above the tie, and not require workers' hands in the hazardous area between an upraised rail and the rail seat.